(1) Field of the Invention
The present invention generally relates to an infinitesimal displacement measuring apparatus and an optical pick-up unit, and more particularly to an infinitesimal displacement measuring apparatus for measuring an infinitesimal displacement of an article and an optical pick-up unit, used in an optical head of an optical disk system such as a magneto-optical disk system, to which the infinitesimal displacement measuring apparatus is applied.
(2) Description of the Related Art
Conventionally, an astigmatism method, a critical angle method and a knife-edge method have been known as methods applicable to a focus servo control system using focus error signals in an optical head of a magneto-optical disk unit. The astigmatism method is applied to not only the magneto-optical disk unit but also other optical disk units such as a compact-disk unit and a video-disk unit. Prior art regarding the astigmatism method is disclosed, for example, in Japanese Patent Publication No.53-39123, No.57-12188, No.60-48949 and No.61-61178.
FIG. 1 shows the principle of the astigmatism method applied to a conventional optical pick-up unit of a magneto-optical disk system.
Referring to FIG. 1, after a laser beam emitted from a semiconductor laser unit (not shown) is collimated by a collimate lens (not shown), the laser beam passes through a beam splitter 1 and is then focused on a surface of an optical disk 3 by an objective lens 2. Under a condition in which the laser beam is focused on the surface of the optical disk 3, a recording operation and a reproduction operation are carried out. A reflected beam from the optical disk 3 is turned back to the beam splitter 1 via the objective lens 2, and is reflected by the beam splitter 1. The beam reflected by the beam splitter 1 travels through a focusing lens 4 and a cylindrical lens 5, so that a beam 6 having the astigmatism is generated. This beam 6 is incident on a surface of a photo-detector 7. The surface of the photo-detector 7 is divided into four divisional detecting areas a, b, c and d. Signals output from the respective divisional detecting areas of the photo-detector 7 are supplied to an amplifier 8, so that focus error signal Fo, depending on a degree of defocusing of the laser beam on the surface of the optical disk 3, is output from the amplifier 8.
In a case where the laser beam is accurately focused on the surface of the optical disk 3, that is, in a case where the optical disk 3 is at a focused position, the beam 6 based on the laser beam reflected by the optical disk 3 is circularly shaped on the four divisional detecting areas a, b, c and d of the photo-detector 7. In this case, a differential output {(a+c)-(b+d)} from the amplifier 8 has a zero level, that is, the focus error signal Fo has the zero level (no focus error sinal is output).
On the other hand, in a case where the laser beam is defocused on the surface of the optical disk 3, that is, in a case where the optical disk 3 is at a position which is nearer to the objective lens 2 than the focused position or at a position which is further from the objective lens 2 than the focused position, the beam 6 is elliptically shaped on the surface of the photo-detector 7. In this case, the differential output from the amplifier 8 is not zero, that is, the defocusing signal Fo has a positive level (further from the objective lens 2) or a negative level (nearer to the objective lens 2). A position of the objective lens 2 is controlled based on the focus error signal Fo so that the laser beam is always accurately focused on the surface of the optical disk 3.
That is, in the above optical pick-up unit, the infinitesimal displacement of the optical disk 3 in a direction of an optical axis of the laser beam is measured, and the objective lens 2 is adjusted, based on the measurement result (the focus error signal Fo), so that the laser beam is always accurately focused on the surface of the optical disk 3.
In the recent years, this type of the optical disk unit is required to improve an access time for information. To comply with this request, the optical pick-up unit of the optical head must be miniaturized and lightened. However, in the above focus servo control system based on such the astigmatism method, since the variation of the spot shape of the laser beam has to be detected, a long optical path (e.g. a few centimeters) from the optical disk 3 to the photo-detector 7 is needed to obtain a sufficient detecting sensitivity. Thus, in the conventional optical disk unit, the miniaturization thereof is limited.
In addition, since the diameter of the laser beam spot on the surface of the photo-detector 7 is very small (e.g. falling within a rage from a few micrometers to a few tens micrometers), it is difficult to adjust a position at which the beam is incident on the photo-detector 7. As a result, the differential output from the amplifier 8 may have an off-set level, so that focus error signals obtained from the differential output are unstable.